Radiotherapy apparatus for delivering radiation to a subject

ABSTRACT

The present application relates to a radiotherapy apparatus for delivering radiation to a subject. The apparatus comprises a source of radiation configured to rotate about an isocenter and emit radiation in a radiation plane containing said isocentre. The apparatus also comprises a subject support surface configured such that a portion of the subject support surface can be located substantially at the isocenter. The apparatus also comprises a subject support surface rotation mechanism configured to rotate the subject support surface about an axis of rotation that passes through the isocenter, wherein the subject support surface rotation mechanism is located outside the radiation plane.

This disclosure relates generally to a radiotherapy apparatus, and inparticular to positioning a subject during the delivery or applicationof radiotherapy.

BACKGROUND

Radiotherapy uses ionising radiation to treat a human or animal body. Inparticular, radiotherapy is commonly used to treat tumours within thehuman or animal body. In such treatments, cells forming part of thetumour are irradiated by ionising radiation in order to destroy ordamage them. However, in order to apply a prescribed dose of ionisingradiation to a target location or target region, such as a tumour, theionising radiation will typically also pass through healthy tissue ofthe human or animal body. Therefore, radiotherapy has the desirableconsequence of irradiating and damaging a target region, but can alsohave the undesirable consequence of irradiating and damaging healthytissue. In radiotherapy treatment, it is desirable to align the dosereceived by the target region with a prescribed dose and to minimise thedose received by healthy tissue.

Modern radiotherapy treatment uses techniques to reduce the radiationdose to healthy tissue and thereby provide a safe treatment. Forexample, one approach to minimising a radiation dose received by healthytissue surrounding a target region is to direct the radiation towardsthe target region from a plurality of different angles, for example byrotating a source of radiation around the patient by use of a rotatinggantry. In this case, the angles at which radiation is applied areselected such that each beam of radiation passes through the targetregion. In this way, a cumulative radiation dose may be built up at thetarget region over the course of a treatment arc in which the radiationsource rotates through a certain angle. Radiation is emitted in aradiation plane which is co-incident with the plane of the gantry aroundwhich the radiation source rotates and radiation may thus be deliveredto a radiation isocentre at the centre of the gantry regardless of theangle to which the radiation head is rotated around the gantry. Becausethe radiation is applied from a plurality of different angles, the same,high, cumulative radiation dose is not built up in the healthy tissuesince the specific healthy tissue the radiation passes through varieswith angle. Therefore, a unit volume of the healthy tissue receives areduced radiation dose relative to a unit volume of the target region.Treatments that utilise rotation of the gantry in this manner are knownas coplanar. However, after the radiation source has been rotated 180°,it will be appreciated that any subsequent radiation beams begin to passthrough regions of healthy tissue which have already been irradiated.This increases the radiation dose applied to healthy tissue.Accordingly, when using such a method the volume of healthy tissueavailable to spread the radiation dose is relatively small, thusimposing restrictions on the treatment which can be provided by suchdevices.

Therefore, an alternative approach to minimising the radiation dosereceived by healthy tissue surrounding a target region is to rotate thepatient relative to the plane of radiation. As the angle of the patientvaries relative to the plane of the gantry, so does the healthy tissuethe radiation passes through. In order to further reduce the radiationdose relative to a unit volume of the target region, it is desirable toprovide a treatment that combines both of these rotations. An example ofa known device that combines the rotation of the patient with therotation of the radiation source is shown in FIG. 1 . This shows thatthe patient 140, who is supported on the subject support surface 114,which is also referred to herein as a patient support surface 114, canbe rotated whilst the gantry 116 may also rotate about the patientsupport surface 114. The gantry 116 shown in FIG. 1 is a C-arm gantry oropen gantry. The rotation mechanism 117 rotates the gantry 116 about afixed axis 119. As the gantry 116 is rotated, radiation emitted by aradiation source 106 can sweep out a circle. Radiation can be applied tothe patient 140 from a plurality of angles around the circle. The circlemay be described as lying in a radiation plane. The radiation axis liesin the radiation plane. The radiation axis makes an angle of 90° withrespect to the fixed axis 119.

The rotation mechanism 120 for the patient support surface 114 islocated underneath the gantry 116 of the radiotherapy device, while arotation mechanism for the gantry 116 is located opposite the patientsupport surface 114. The rotation mechanism 120 for the patient supportsurface 114 is located underneath the gantry 116 so that the axis ofrotation of the patient support surface 111 will be in the radiationplane. In particular, the axis of rotation of the patient supportsurface 111 passes through the isocenter 124 of the radiotherapy device,so that the patient support surface 114 is rotated about the isocenter124. When the patient support surface 114 is in its neural position, theaxis of rotation of the patient support surface 111 is substantiallyvertical (perpendicular to the plane of the floor) and this can also becalled a vertical axis 111. The longitudinal axis 113 is parallel tolong side of the patient support surface 114 in its neutral position andthe transverse axis 115 is parallel to the short end of the patientsupport surface 114 in its neutral position. The rotation mechanism 120is located within the plane of radiation. Treatments utilising both therotation of the radiation and the patient 140 are known as non-coplanartreatments.

Some recently developed radiotherapy devices comprise ring-basedgantries (or bores), such as that shown in FIG. 2 . Typically, the boreof a radiotherapy device is cylindrical. A patient support surface 114is positioned in the bore such that radiation can be directed toward apatient 140 positioned on the support surface 114. The bore of theapparatus can be formed by a framework, which may otherwise be describedas a chassis, a shielding structure, a shell, or a casing. The frameworkdefines the outer surface of the device which the patient 140 sees uponentering the treatment room, as well as defining the inner surface ofthe bore which the patient 140 sees when positioned inside the bore. Theframework also defines a hollow region of annular cross-section in whichthe gantry 116 can be both rotated and tilted. Thus, the patient 140 isshielded from the rotatable gantry 116. Movement of the gantry 116 ishidden from the patient's view, reducing intimidation and distress whichmay otherwise be caused if the patient 140 were able to see rotation ofthe large gantry 116, as they would for an open gantry as shown in FIG.1 , and also reducing the likelihood that the patient can accidentallytouch or otherwise interfere with the movement of the gantry 116. Thismeans that the gantry 116 can be rotated quickly, efficiently andsafely. Ring-based gantries are also desirable because they increasedevice stability. The ring-based gantry is supported by the floor andrests upon it. However, the geometry of a ring-based gantry and itsconnection to the floor makes it impossible to rotate the patientsupport surface 114 about the isocenter 124.

Another problem that arises when attempting to minimise the radiationdose received by healthy tissue surrounding a target region can be foundin accurately locating the position of the target region relative to thedevice. For example, movement of the patient can cause movement ofunhealthy tissue such as a tumour and thus the dose applied to thetarget region may be decreased and the dose applied to the healthytissue may be increased. In other words, if a patient moves during orprior to radiotherapy treatment, this can cause a high cumulative doseto build up in a region of healthy tissue instead of in a target region.This can reduce the effectiveness of the radiotherapy for treating thetarget region and can cause damage to otherwise healthy tissue.

This problem is also caused by flexing of the table top of the patientsupport surface when the table top is extended into the device. Innormal operation, the table top will initially be positionedsubstantially outside the plane of the gantry to enable the patient toeasily position themselves onto the table. The table top will then beextended into the plane of the gantry and, in particular, such that thatthe target region is aligned with the isocenter of the device. In theextended position, the table top will flex with a magnitude dependent onthe position of the table top, the position of the patient on the tabletop and the weight of the patient. Due to the table top flex, the targetregion will move relative to the isocenter and this will result inhealthy tissue receiving a higher dose of radiation than is necessary.Furthermore, during spiral treatments, which are used to target a largertarget region, the table top is moved during the treatment. Spiraltreatments involve the patient being moved, by movement of a table top,whilst the radiation source moves around the gantry and emits radiation.Accordingly, the amount of table top flex will vary during the treatmentand so the position of the target region relative to the isocenter willvary during the treatment, resulting in healthy tissue receiving ahigher dose of radiation than is necessary. This problem occurs for allradiotherapy devices with an extendable table top. However, this problemis particularly significant for radiotherapy devices with a boresolution, because these devices will often have a longer top extension.The longer the table top extension, the more table top flex will occurand the greater will be the change in position of the target region.

Previous solutions to this problem involve manually positioning thepatient, for example with the assistance of lasers. However,particularly for automatic and spiral treatments this does not workwithout repeatedly stopping the treatment and thereby resulting inlonger treatment times and lower efficiency. It is possible to detectthe position of the target region by taking an image, however doing sois harmful to the patient. It would be desirable to know the position ofthe treatment region as accurately as possible at all times during thetreatment without the need to take additional images. Accurately knowingthe position of the target region allows the radiation to be focusedwhere it is needed, ideally within 1 mm of the target region, therebyminimising the radiation dose received by the healthy tissue surroundingthe target region.

SUMMARY

An invention is set out in the claims.

FIGURES

Specific embodiments are now described, by way of example only, withreference to the drawings, in which:

FIG. 1 depicts a known radiotherapy apparatus with rotation meanslocated within the plane of the radiation;

FIG. 2 depicts a front view of a radiotherapy device;

FIG. 3 depicts an isometric view of an embodiment of the radiotherapydevice;

FIG. 4A depicts an isometric view of an embodiment of the radiotherapydevice comprising a curved rail;

FIG. 4B depicts a plan view of an embodiment of the radiotherapy devicecomprising a curved rail;

FIG. 5 depicts a side elevation view of an embodiment of theradiotherapy device comprising a curved rail, with an unloaded subjectsupport surface in its non-extended position;

FIG. 6 depicts an isometric view of an embodiment of the radiotherapydevice comprising two curved rails aligned vertically;

FIG. 7 depicts an isometric view of an embodiment of the radiotherapydevice connected to the floor, the radiotherapy device comprising twocurved rails aligned vertically;

FIG. 8 depicts a detailed isometric view of an embodiment of theradiotherapy device comprising two curved rails aligned vertically;

FIG. 9 depicts a side elevation view of an embodiment of theradiotherapy device and illustrates bending of an extendable table topof a patient support surface that is loaded and in its extendedposition;

FIG. 10A depicts a side elevation view of an embodiment of theradiotherapy device and illustrates a subject support surface in itsloaded and non-extended position;

FIG. 10B depicts a close up of a side elevation view of an embodiment ofthe radiotherapy device and illustrates a position of a sensor withrespect to the subject support surface;

FIG. 11 depicts a side elevation view of an embodiment of theradiotherapy device and illustrates a subject support surface in itsloaded and extended position.

OVERVIEW

By providing a radiotherapy apparatus for delivering radiation to asubject, with the apparatus comprising a subject support surfaceconfigured such that a portion of the subject support surface can belocated substantially at the isocenter and a subject support surfacerotation mechanism configured to rotate the subject support surfaceabout an axis of rotation that passes through the isocenter, wherein thesubject support surface rotation mechanism is located outside theradiation plane, a number of benefits are provided. The apparatusprovides means for allowing the dose received by healthy tissue during aradiotherapy treatment to be minimised. By rotating the subject supportsurface, for example with a patient positioned on it, it is possible tospread the radiation through the healthy tissue while rotating about theisocenter ensures that the maximum amount of radiation still passesthrough the target region which maximises the efficiency of thetreatment and allows the treatment time to be reduced. However, if inorder to rotate a couch about the isocenter, the rotation mechanism islocated within the plane of the radiation, then it is not possible touse the couch and rotation mechanism in radiotherapy device using a borebecause the gantry and the rotation mechanism would obstruct oneanother. Locating the subject support surface rotation mechanism outsidethe radiation plane allows the dose received by healthy tissue of thesubject during the radiotherapy treatment to be minimised for a widerange of radiotherapy apparatuses with different geometries. Forexample, these benefits can be achieved in radiotherapy apparatuses thatcomprise a bore for receiving the subject.

By providing a system for positioning a subject in a radiotherapyapparatus, with the system comprising a subject support surface with anextendable table top and one or more sensors that are configured tomeasure a vertical position of the extendable table top, as well as aprocessor configured to determine a deflection of the extendable tabletop using the measured position and control a treatment of theradiotherapy apparatus according to the deflection, a number of benefitsare provided. Using this system to determine a deflection profile allowstreatments, such as spiral treatments for example, to be performedaccurately without the need for re-imaging the patient during treatment.This reduces the amount of imaging radiation that the patient is exposedto which reduces the harm done to a patient. Removing the requirementfor re-imaging increases the speed at which a treatment can beperformed, thereby increasing patient throughput and improving theefficiency of the radiotherapy apparatus. The system also enables theposition of the target region to be known with greater certainty andaccuracy, which enables the treatment to be performed with greateraccuracy and confidence in treating the target region. This alsominimises the radiation received by healthy tissue.

DETAILED DESCRIPTION

When administering a treatment to a subject or patient 140 with aradiotherapy apparatus comprising a source of radiation 106 configuredto rotate about an isocenter 124 and emit radiation in a radiation planecontaining said isocentre 124, rotating the subject about the isocenter124 allows the dose received by healthy tissue during the radiotherapytreatment to be minimised. This can be achieved by providing a subjectsupport surface rotation mechanism 120 connected to the subject supportsurface 114 and configured to rotate the subject support surface aboutthe isocenter 124. Rotating the subject support surface 114 about anaxis of rotation that passes through the isocenter 124 ensures that theradiation will pass through the same point, regardless of the rotationangle of the subject support surface 114. This is advantageous because,for example, by locating a target region of a patient 140 at theisocenter 124, it is possible to ensure that the radiation passesthrough the target region for all rotation angels of the subject supportsurface 114. By rotating the subject support surface 140 (and thereforea patient 140), it is possible to spread the radiation through thehealthy tissue while rotating about the isocenter 124 ensures that themaximum amount of radiation still passes through the target region whichmaximises the efficiency of the treatment and allows the treatment timeto be reduced. Locating the subject support surface rotation mechanism120 outside the radiation plane allows the dose received by healthytissue of the subject 140 during the radiotherapy treatment to beminimised for a wide range of radiotherapy apparatuses with differentgeometries. In particular, radiotherapy apparatuses that comprise a borefor receiving the subject 140. By way of background, in known devicesthe rotation mechanism is located within the plane of radiation, asshown in FIG. 1 , which make them unsuitable for radiotherapyapparatuses that comprise a bore.

In accordance with one embodiment, FIG. 3 depicts a radiotherapy devicesuitable for delivering a beam of radiation to a patient duringradiotherapy treatment. The device and its constituent components willbe described generally for the purpose of providing useful accompanyinginformation for the present invention. The device depicted in FIG. 3 isin accordance with the present disclosure and is suitable for use withthe disclosed systems and apparatuses, although not all of the featuresare necessarily present, or as depicted in FIG. 3 . While the device inFIG. 3 is an MR-linac, the implementations of the present disclosure maybe any radiotherapy device, for example a linac device. FIG. 3 sharesfeatures common with known devices such as Versa HD™ in particular, thefeatures involved in producing the treatment beam 110. The embodimentshown in FIG. 3 is modified over known devices in accordance with theinvention by the provision of a subject support surface rotationmechanism 120, as will be described in more detail below.

The device depicted in FIG. 3 is an MR-linac. The device comprises bothMR imaging apparatus 112 and radiotherapy (RT) apparatus which maycomprise a linac device. In operation, the MR scanner produces MR imagesof the patient 140, which can be used to determine the position of thepatient 140 on the couch 114 and also the position of a target region,such as a tumour, within the patient 140 so that a target region'sposition relative to the couch 114 may be determined. The linac deviceproduces and shapes a beam of radiation and directs it toward a targetregion within a patient's body in accordance with a radiotherapytreatment plan. The usual ‘housing’ which would cover the MR imagingapparatus 112 and RT apparatus in a commercial setting such as ahospital is not depicted in FIG. 3 .

The MR-linac device depicted in FIG. 3 comprises a source of radiation106. The source of radiation 106 may comprise beam generation equipment,such as one or more of: a source of radiofrequency waves 102, acirculator 118, a source of electrons 105, a waveguide 104, and a target(not shown). The MR-linac may also comprise a collimator 108 such as amulti-leaf collimator configured to collimate and shape the beam, MRimaging apparatus 112, and a patient support surface 114. The devicealso comprises a housing which, together with the ring-shaped gantrydefines a bore. The movable subject support surface 114 can be used tomove a patient, or other subject, into the bore when an MR scan and/orwhen radiotherapy is to commence or during treatment. The MR imagingapparatus 112, RT apparatus, and a subject support surface actuator arecommunicatively coupled to a controller or processor. The controller isalso communicatively coupled to a memory device comprisingcomputer-executable instructions which may be executed by thecontroller.

The RT apparatus comprises a source of radiation 106 and a radiationdetector (not shown). Typically, the radiation detector is positioneddiametrically opposed to the radiation source 106. The radiationdetector is suitable for, and configured to, produce radiation intensitydata. In particular, the radiation detector is positioned and configuredto detect the intensity of radiation which has passed through thesubject. The radiation detector may also be described as radiationdetecting means, and may form part of a portal imaging system.

The radiation source 106 defines the point at which the treatment beam110 is introduced into the bore. The radiation source 106 may comprise abeam generation system, which may comprise a source of RF energy 102, anelectron gun 105, and a waveguide 104. The beam generation system isattached to the rotatable gantry 116 so as to rotate with the gantry116. In this way, the radiation source 106 is rotatable around thepatient 140 so that the treatment beam 110 can be applied from differentangles around the gantry 116. In a preferred implementation, the gantry116 is continuously rotatable. In other words, the gantry 116 can berotated by 360 degrees around the patient, and in fact can continue tobe rotated past 360 degrees. The gantry 116 rotates about a mechanicalisocenter, which is the point in space about which the gantry 116rotates and about a fixed axis 119. The radiation isocenter can bedefined as the point where the radiation beams intersect. These twoisocenters 124 need not be the same, although they can be. In thisdisclosure, the term isocenter 124 can refer to either or both of these.The isocenter 124 is located within the radiation plane. The gantry 116may be ring-shaped. In other words, the gantry 116 may be a ring-gantrywith a bore. The gantry 116 may also not be ring-shaped and may insteadbe an open gantry such as that shown in FIG. 1 .

The source 102 of radiofrequency waves, such as a magnetron, isconfigured to produce radiofrequency waves. The source 102 ofradiofrequency waves is coupled to the waveguide 104 via circulator 118,and is configured to pulse radiofrequency waves into the waveguide 104.Radiofrequency waves may pass from the source 102 of radiofrequencywaves through an RF input window and into an RF input connecting pipe ortube. A source of electrons 105, such as an electron gun, is alsocoupled to the waveguide 104 and is configured to inject electrons intothe waveguide 104. In the source of electrons, electrons arethermionically emitted from a cathode filament as the filament isheated. The temperature of the filament controls the number of electronsinjected. The injection of electrons into the waveguide 104 issynchronised with the pumping of the radiofrequency waves into thewaveguide 104. The design and operation of the radiofrequency wavesource 102, electron source and the waveguide 104 is such that theradiofrequency waves accelerate the electrons to very high energies asthe electrons propagate through the waveguide 104.

The source of radiation 106 is configured to direct a beam 110 oftherapeutic radiation toward a patient positioned on the patient supportsurface 114. The source of radiation 106 may comprise a heavy metaltarget toward which the high energy electrons exiting the waveguide aredirected. When the electrons strike the target, X-rays are produced in avariety of directions. A primary collimator may block X-rays travelingin certain directions and pass only forward traveling X-rays to producea treatment beam 110. The X-rays may be filtered and may pass throughone or more ion chambers for dose measuring. The beam can be shaped invarious ways by beam-shaping apparatus, for example by using amulti-leaf collimator 108, before it passes into the patient as part ofradiotherapy treatment.

In some implementations, the source of radiation 106 is configured toemit either an X-ray beam or an electron particle beam. Suchimplementations allow the device to provide electron beam therapy, i.e.a type of external beam therapy where electrons, rather than X-rays, aredirected toward the target region. It is possible to ‘swap’ between afirst mode in which X-rays are emitted and a second mode in whichelectrons are emitted by adjusting the components of the linac. Inessence, it is possible to swap between the first and second mode bymoving the heavy metal target in or out of the electron beam path andreplacing it with a so-called ‘electron window’. The electron window issubstantially transparent to electrons and allows electrons to exit theflight tube.

The radiotherapy apparatus/device depicted in FIG. 3 also comprises MRimaging apparatus 112. The MR imaging apparatus 112 is configured toobtain images of a subject positioned, i.e. located, on the subjectsupport surface 114. The MR imaging apparatus 112 may also be referredto as the MR imager. The MR imaging apparatus 112 may be a conventionalMR imaging apparatus 110 operating in a known manner to obtain MR data,for example MR images. The skilled person will appreciate that such a MRimaging apparatus 112 may comprise a primary magnet, one or moregradient coils, one or more receive coils, and an RF pulse applicator.The operation of the MR imaging apparatus is controlled by thecontroller.

The controller is a computer, processor, or other processing apparatus.The controller may be formed by several discrete processors; forexample, the controller may comprise an MR imaging apparatus processor,which controls the MR imaging apparatus 112; an RT apparatus processor,which controls the operation of the RT apparatus; and a subject supportsurface processor which controls the operation and actuation of thesubject support surface. The controller is communicatively coupled to amemory, i.e. a computer readable medium.

The linac device also comprises several other components and systems aswill be understood by the skilled person. For example, in order toensure the linac does not leak radiation, appropriate shielding is alsoprovided.

The patient support surface 114 may serve to support an object. Theobject may be a human body (such as a patient), an animal body or amaterial sample. The subject support surface 114 is configured to moveparallel to the longitudinal axis 113 between a first positionsubstantially outside the bore, and a second position substantiallyinside the bore. In the first position, a patient 140 or subject canmount the subject support surface 114. The subject support surface 114,and patient 140, can then be extended inside the bore, to the secondposition, in order for the patient 140 to be imaged by the MR imagingapparatus 112 and/or imaged or treated using the RT apparatus. Themovement of the subject support surface 114 is effected and controlledby a subject support surface actuator, which may be described as anactuation mechanism. The actuation mechanism is configured to move thesubject support surface 114 in a direction parallel to, and defined by,the longitudinal axis of the subject support surface 114. The termssubject and patient are used interchangeably herein such that thesubject support surface 114 can also be described as a patient supportsurface 114. The subject support surface 114 may also be referred to asa movable or adjustable couch or table.

The present invention is distinguished over known devices as follows.The subject support surface 114 is connected to a subject supportsurface rotation mechanism 120. The rotation mechanism 120 can beattached to the floor as shown or, for example, can be attached to thedevice housing or gantry 116 (as shown in, for example, FIG. 4A). Therotation mechanism 120 is configured to rotate the patient supportsurface 114 with the axis of rotation of the patient support surface 114passing through the isocentre 124 of the gantry 116. The patient supportsurface 114 or part thereof can be rotated around (or about) thelongitudinal axis 113 (roll), around the transverse axis 115 (pitch), orabout an axis perpendicular to the floor 111 (yaw), or any combinationof these.

Although in FIG. 3 the plane of the rotation of the patient supportsurface 114 is illustrated as being parallel to the illustrated floor(as is defined by the xy plane, which corresponds to the plane of thepatient support surface 114 in its neutral position where x is thelongitudinal axis 113 and y is the transverse axis 115), with rotationas yaw about the axis 111, by way of example, the angle of the plane ofrotation relative to the floor could be at an angle of 3, 15, 45 or 90degrees to the floor. However, for reasons of patient comfort, the anglewill usually be kept fairly low. It is also possible for the tilt to bechanged either prior to, or during, treatment. The subject supportsurface rotation mechanism is configured to rotate the subject supportsurface +/−40-20 degrees about the subject support surface axis ofrotation, more preferably 35-25 degrees, most preferably 30 degrees. Therotation mechanism 120 and/or the patient support surface 114 may alsobe connected to an additional rotation mechanism (not shown) configuredto rotate the rotation mechanism 120 and/or the patient support surface114 in a different plane, wherein the axis of rotation also passesthrough the isocenter 124. In this way, the patient support surface 114may be connected to more than one rotation mechanism, each configured tomove the patient support surface 114 in a different plane.Alternatively, a single rotation mechanism 120 may be configured torotate the patient support surface 114 in more than one plane with theaxis of rotation of each of the rotation planes of the patient supportsurface 114 passing through the isocenter 124. The primary considerationis that the centre of rotation (about whichever axis) is located at theisocenter 124, or close to the isocenter 124. As a result, the treatmentbeam can be consistently focused on the area requiring treatment.

By rotating the couch and hence the patient around the isocenter, theradiation dose can be spread through the healthy tissue so that theradiation dose received by healthy tissue surrounding a target region isminimised. This improves patient 140 wellbeing. If the rotation of thecouch 114 was not about the isocenter 124, then the location of thetarget region would move with respect to the isocenter 124 (and focus ofthe radiation) and, accordingly, this would result in an increaseddosage of radiation being received by healthy tissue. Furthermore, thiswould result in a longer treatment time because the target region wouldnot receive the intended dosage of radiation.

The disclosure provides rotation means for rotating a patient supportsurface around an isocenter 124, whilst locating the patient supportsurface rotation mechanisms 120 outside the radiation plane. This isparticularly useful for ring gantry/bore solutions or devices with 360°rotation of the gantry 116, for which it is problematic to position therotation mechanism 120 within the radiation plane without interferingwith the gantry 116. However, this disclosure is applicable to anyradiotherapy device. Whilst the disclosure is not limited to boresolutions (ring gantries), bore solutions offer improved devicestability. Furthermore, bore solutions are less imposing or alarming forpatients. Bore solutions therefore may be desirable. The disclosureprovides means to supply non-coplanar treatments (in which both gantry116 and patient support surface 114 are rotated) in a radiotherapydevice with a bore solution.

The disclosure provides rotation means which are located outside of theplane of the gantry 116 and therefore the plane of radiation, orisoline. Positioning the rotation means 120 outside the plane ofradiation minimises radiation interference.

Examples of specific linkages and structures for rotating a subjectsupport surface about an axis of rotation that passes through theisocenter, wherein the subject support surface rotation mechanism islocated outside the radiation plane, will now be described.

One embodiment is shown from three different perspectives in FIGS. 4A,4B and 5 . These figures show a patient support surface 114 (which mayalso be described as a couch or patient positioning system) supported byand connected to a rotation mechanism 120. The couch 114 is connecteddirectly to the rotation mechanism 120 or via an intermediary and can beconnected by any suitable means, for example, mechanically. The couch114 may include a number of rollers, a table top 144, a table base, orother parts. In these figures, the rotation mechanism 120 is connectedto the gantry 116 but it could be connected to a floor, a wall or othersupport structure instead or as well. The rotation mechanism 120 shownhere makes use of two curved guides or rails 122, with the centre ofcurvature for both curved guides, which may be curved guide rails 122being located substantially at the isocenter 124. For example, thecenter of curvature (and the center of rotation of the patient supportsurface 114) could be within 0.005 to 0.015 mm, more preferably 0.01 mm,0.05 mm to 0.15 mm, more preferably 0.1 mm, 0.15 mm to 0.25 mm, morepreferably 0.2 mm, 0.25 to 0.35 mm, more preferably 0.3 mm, 0.35 mm to0.45 mm, more preferably 0.4 mm, 0.45 mm to 0.55 mm, more preferably 0.5mm, 0.5 mm to 1.5 mm, more preferably 1 mm, or another distance of theisocenter 124. Ideally, the centre of curvature of each curved rails 122and the center of rotation of the couch 114 will be as close to theisocenter 124 as possible. There could be one curved rail 122 or anylarger number. In one example in which there are two curved rails 122,both of the curved rails 122 have the same radius. In another example,the two curved rails 122 have different radii but, the centre ofcurvature for both of the curved rails 122 is still the same.

The rotation mechanism 120 itself can be moved up and down in anydirection, such as vertically as shown in FIG. 4A. The patient supportsurface 114 can move in any direction. Alternatively, or as well, thepatient support surface 114 may comprise a table top 144 which can moveindependently from the rest of the patient support surface 114, such asa table base, and in any direction, for example, a longitudinaldirection (along a longitudinal axis 113 of the patient support surface114), a lateral direction (along a transverse axis 113 of the patientsupport surface 114), a vertical direction (along a vertical axis 111that is an axis perpendicular to the floor), or a direction oblique toany of these directions. In some embodiments, the longitudinal direction113 may be described as Y direction 113. The lateral or transversedirection 115 may be described as the X direction 115. The verticaldirection 111 may be described as the Z direction 111. The rotational,vertical or other movements can be driven manually or by, for example,one or more motors.

In the example illustrated in FIGS. 4A, 4B and 5 , the plane of rotationof the couch 114 is shown as being parallel to the floor. This maycommonly be the case but it is not limited to this. The curved rails 122themselves may be fixed at an incline to the floor or the tilt mayactually be altered before, during or after the rotation of the couch114. Furthermore, the couch 114 may comprise a table top 144 which isitself configured to rotate, for example about the axis of the bore orthe longitudinal axis 113. This also serves to minimise the radiationdose received by healthy tissue surrounding a target region.

The rotation of the patient support system 114 can occur before, duringor after treatment. Rotation can be continuous or discrete/static.Rotation of the couch 114 may also occur with the table top 144 extendedor not extended. Rotation of the couch 114 can also occur at the sametime as table top 144 is being extended. In one example, a patient 140lies on the couch 114 in its non-extended position. The couch 114 isthen extended, the patient is scanned and exposed to radiation. Theradiation is then stopped, the couch 114 is rotated (yawed) manually bysliding the couch 114 along the curved rail 122 and the patient is thenexposed to further radiation. In another example, the radiation is notstopped and the rotation of the couch 114 happens automatically and atthe same time as the patient is exposed to radiation.

This rotation may be controller by a processor which may be comprised inthe patient support surface 114 or may be found elsewhere. For example,the processor can control the speed of rotation, the angle of rotationor the amount of rotation. This processor may also be used to controlthe radiation emission, radiotherapy treatment or other operation of theradiotherapy device. This can allow the rotation of the couch 114 to besynchronized with the operation of the radiotherapy device or deliveryof the radiotherapy treatment.

In a bore solution, such as that shown in FIGS. 4 and 4A, the rotationof the couch 114 may be inhibited at some angles by the gantry 116 organtry cover. For example, the couch 114 may be rotated by +/−30° fromthe neutral position. The neutral position is when the couch 114 isaligned with the axis of the bore and parallel to the floor. When thepatient support system 114 is fully extended into the bore, there may beless rotation possible compared to when the patient support system 114is not extended, or only partially extended, into the bore. As a result,this system is particularly well suited to treatments for head and neck.

The one or more curved rails 122 may be made from the same materials ordifferent materials. For example, each curved rail 122 could be madefrom a metal, for example steel. The curved rails 122 can be fixed tothe floor or another support surface. The rails comprise a track andslider. The slider can be attached to the table top or on the frame. Theslide position can be controlled by a linear motor, timing belt ordirect drive. A direct drive is a separate cog track or an integratedcog track onto the rail. To help prevent the sliders crabbing betweenthe rails, some flexures can be used to compensate tolerances.

Alternatively, the rotation mechanism 120 may not in fact comprise acurved rail 122 but may comprise one or more curved trenches, with thecenter of curvature of the one or more curved trenches substantiallylocated at the isocenter 124, wherein the one or more curved trenchesserves to guide the rotation of the patient support surface 114 aboutthe isocenter 124. Alternatively, the rotation mechanism 120 maycomprise one or more curved rails 122 and one or more curved trenches,with the center of curvature of both substantially located at theisocenter 122. Where the centre of curvature is referred to as beingsubstantially located at the isocenter 122, this includes any point thatfalls substantially along a vertical axis passing through the isocenter122, as well as the isocenter 122 itself. Accordingly, it will beapparent that the particular means used to guide the rotation can bevaried and the important concept is that the centre of curvature of therotation guide is located substantially at the isocenter 124.

The radiation source or gantry 116 itself may also be partially rotatedabout the transverse axis of the short end of the patient supportsurface 114 in its neutral position, although not necessary when thepatient support surface 114 is in its neutral position, either at thesame time, or a different time, synchronously or separately to thepatient support surface 114.

By using a rotation mechanism 120 comprising curved rails as described,it is possible to cause pure isocenter rotation of the patient supportsystem 114 without the rotation mechanism 120 sharing a commonmechanical axis with the gantry 116. In other words, isocenter rotationand the benefits that come with that are achieved whilst keeping therotation mechanism 120 outside the radiation unit, thereby notinterfering with gantry rotation or interfering with the delivery of theradiation. Accordingly, the present disclosure allows the dose receivedby healthy tissue during radiotherapy treatment to be minimised.

Another embodiment is shown from three different perspectives in FIGS.6, 7 and 8 . These figures show a patient support surface 114 supportedby and connected to a rotation mechanism 120. The couch 114 is connecteddirectly to the rotation mechanism 120 or via an intermediary and can beconnected by any suitable means, for example, mechanically. The couch114 may include a number of rollers, a table top 144, a table base, orother parts. In these figures, the rotation mechanism 120 is connectedto the floor and, in particular, within a pit 121 that forms part of thefloor but it could be connected to a gantry 116, a wall or other supportstructure instead or as well. The rotation mechanism 120 is shown asbeing partly comprised within the pit 121, but may be formed completelyinside the pit 121. The rotation mechanism shown in FIGS. 6, 7 and 8 issimilar to the rotation mechanism shown in FIGS. 4A, 4B and 5 , asdiscussed above. For example, the rotation mechanism 120 makes use oftwo curved rails 122, with the centre of curvature for both curved rails122 being located substantially at the isocenter 124. However, in thisembodiment, the curved rails 122 are stacked on top of each other, whichis to say that they are parallel with one another but spaced apart fromone another in a vertical direction, for example along a vertical axis.The vertical axis 111 is the axis of rotation. The rotation mechanism120 is shown as being comprised within a box 123.

In this embodiment, the couch 114 is connected to the curved rails 122by an arm 125. The arm 125 may be connected to the curved rails 122using any appropriate means. For example, the arm 125 may comprise firstand second slots for the first and second curved rails 122 to engagewith. The first and second slots may be straight or may be curved with aradius of curvature designed to match the radius of curvature of thecurved rails 122. In this example, the arm 125 also acts as the base forthe couch 114 but the arm 125 may be separate from the base of the couch114. In one example, instead of using curved rails 122, curved trenchesare used. Any other appropriate curved guide may also be used instead ofthe curved rails 122 referred to in this disclosure. In another example,a curved trench is used in conjunction with a curved rail 122, both ofwhich have a centre of curvature that is the same and that is located atthe isocenter 124 (or a point along a vertical axis passing through theisocenter 124). In another example, the curved rail 122 has a differentradius to the curved slot but, the centre of curvature for both of thecurved rail 122 and the curved slot is still the same. By separatingcurved rails 122 (or a curved rail 122 and a curved trench) vertically,the rotation mechanism 120 can be kept compact, thereby savinghorizontal space.

As described above, the patient support surface 114 may comprise anextendable table top 144 which can move independently from the rest ofthe patient support surface 114, such as a table base, and in anydirection, for example, a longitudinal direction (along a longitudinalaxis 113 of the patient support surface 114). This can be extended froma first position, for example, a position outside the plane of thegantry 116 (as shown in FIG. 9A) to a second position, for example, aposition that results in a portion of the couch 114 or table top 144being inside the plane of the gantry 116 (as shown in FIG. 11 ). Thisserves multiple purposes which include enabling a patient 140 to easilyclimb onto the couch 114 when it is in a first position, beforepositioning the patient 140 so as to receive the treatment beam 110 inthe second position. This extension can also be done to compensate forthe movement of the couch 114. This extension can also be performed aspart of a spiral treatment, as described in the background section.

As illustrated in FIG. 9 , when the table top 144 is in the second(extended) position, the weight of the patient 140, the table top 144itself or both of these weights, cause the table top 144 to flex (alsointerchangeably referred to herein as bend or deflect). The amount offlex depicted in FIG. 9 is exaggerated for illustrative purposes. Anembodiment of the invention provides a system that allows this table top144 bending to be compensated for in such a way as to enable theradiation to be more accurately focused on the position of the targetregion, as shall be explained by reference to the structure andoperation of the system below.

FIGS. 10A, 10B and 11 show a system for positioning a subject, such as apatient 140, in a radiotherapy apparatus. The radiotherapy apparatus issimilar to that described above in relation to FIG. 3 , however, thesubject support surface 114 also comprises one or more sensors 146configured to measure a vertical position of the table top 144. In oneembodiment, as depicted in FIGS. 10A, 10B and 11 , the subject supportsurface 114 is connected to the gantry 116 and also comprises therotation mechanism 120 (in this case one or more curved rails 122)within the subject support surface 114, although it is not necessary forit to comprise any rotation mechanism 120. In this way and in relationto these Figs., the subject support surface 114 refers to everything,including a rotation mechanism 120 (if present), that supports andpositions the table top 144 in such a way as to position a subject 140in such a way as to receive the treatment beam 110. The subject supportsurface 114 may not be connected to the gantry 116 and be connected to asupport surface such as a floor instead. As described previously, thetable top 144 can be extended along the longitudinal axis 113 using oneor more motors, which can be electric motors with absolute encoders orother encoders, although any other suitable drive mechanism can be usedinstead of one or more of the one or more motors. The table top 144itself is supported in exactly the same way in the outer (first,non-extended) position and the inner (second, extended) position, sothat the absolute table top 144 flex will be the same over the fullstroke (the full range of the extension of the table top 144).

FIG. 10B shows a magnified view of the area comprising the sensor 146.The sensor 146 is located close to the entry of the bore of the gantry116. Although the embodiment depicted in FIGS. 9, 10A, 10B and 11 is ofa radiotherapy apparatus with a bore, the system can also not have abore and can instead have an open gantry.

The sensor 146 is communicatively coupled to a processor and isconfigured to send data to the processor either directly or indirectly.In one example, the sensor 146 comprises a linear variable differentialtransformer (LVDT) that is configured to convert mechanical motion intoan electrical current. LVDT sensors are a known technology, the mode ofoperation of which will not be described here in great detail. However,physically, the LVDT construction is a hollow metallic cylinder in whicha shaft of a smaller diameter moves freely along the cylinder's longaxis. The sensor 146 also comprises a pressure wheel that contacts theunderside of the table top 144 in the first position, in the secondposition, and at all times in between these positions. As the table top144 is extended it flexes, as already described above. This results inthe pressure wheel being compressed, causing the shaft of the LVDT withthe smaller diameter to move inside the larger cylinder which in turncauses an electrical current that corresponds to the displacement of onecylinder relative to the other. In this way, the sensor 146 is used tomeasure a deflection of the table top 144. Other appropriate sensors canalso be used. In particular, other sensors that are known for providingaccurate and easy measurements. For example, an alternative sensor couldbe a laser triangular measurement device of a type well known to theskilled person, so long as it is radiation hard due to the sensor'sproximity to the beam in the scatter area. Alternatively, the sensorcould comprise one or more ultrasonic sensors. The compression of thesensor 146 is related to a position of the table top 144 which in turnis related to a deflection or bend of the table top 144. Any of thesevalues or an electrical signal that can be used to calculate any ofthese values, is then communicated to the processor so that theprocessor can determine the deflection of the table top 146.

The sensor 146 is comprised in the subject support surface 114 so as toonly measure table top 144 flex. The sensor 146 is located outside theimaging/radiation volume and is attached to the couch 114 so that, whenmoving the patient 140 together with the table top 144 into the bore thestructural flex of the rest of the couch 114, is not taken into account.In this way, only the table top 144 flex is measured. Whilst only onesensor 146 is depicted, it should be understood that more than onesensor 146 can be used, for example, to provide redundancy. The samelateral position is measured to avoid any variation in measurementscaused by lateral motion and/or unsmooth underside of the table top 144.

When the table top 144 is in the second longitudinal position (which isan extended position), there is less bending measured compared to in thefirst longitudinal position (which is a non-extended position). Theamount of bending will be increased at both longitudinal positions whenthe table top 144 is loaded, i.e. a patient 140 is positioned on thetable top 144. In the loaded state, the weight of the patient increasesthe bending moment on the table top 144. The table top 144 iseffectively a cantilever, supported at two points towards one end of thetable top. The bending moment will be zero at the free end and it willbe maximum towards the supported end. By measuring a deflection of thetable top 144 in its first position and also in its second position, therelative deflection, change in deflection and/or increase in deflectioncan be determined. It should be understood that the deflection is avalue that is equivalent to a relative position and is calculated basedon a change in the position of the table top 144, in the vertical 111direction, at the location of sensor 146 along the longitudinal axis 113between an unloaded reference state and a loaded state and/or between afirst position and a second position or, as will be explained below.Furthermore, whilst for simplicity the deflection is discussed here inrelation to a first position and a second position, the position of thetable top 144 can be measured at more than two positions. For example,the position of the table top 144 can be measured at 5, 10, 100, 1000 orsome other number of positions along the extension of the table top 144along the longitudinal axis 113. As another example, the position of thetable top 144 can be measured at different levels of extension along thelongitudinal axis 113, for example, every 50 mm, 10 mm, 1 mm or otherinterval. The position of the table top 144 in the vertical direction,relative to the first position (which is also referred to as thedeflection) can therefore be determined for a number of differentpositions, each corresponding to a particular extension of the table top144 on a scale from no extension to full or maximum extension, which mayrefer to the maximum extension used for a particular treatment ratherthan to a maximum possible extension. In one example, the measurementzone is the full treatment zone and the deflection is measured in atleast three different places to determine the tilt angle of the tabletop 144. When the treatment zone is longer, more measurements can betaken because the tilt angle will vary.

In operation, a patient 140 climbs on to the subject support system 114when the table top 144 is in its non-extended position. The table top144, with the patient 140 on it, is then extended into the bore and thetable top 144 deflection is then measured at several positions duringthe transport of the table top 144 into the bore. The amount ofdeflection (or the vertical position of the table top 144 at thelocation of the sensor 146) is measured by the sensor 146 at eachposition and is saved in a memory associated with the processor. In thisway, a deflection profile can be generated and recorded by theprocessor. For example, the profile may resemble a portion of a negativeexponential curve or some other shape, as illustrated in FIG. 9 .

The subject 140 is then imaged to locate the position of the targetregion, so that the treatment beam 110 can be focused on this area. Thesubject support surface 114 can also be controlled and extended in thevertical 111 or lateral 115 direction so that the target region islocated at or close to the radiation isocenter 124 or other desiredlocation.

A spiral treatment is then performed in which the table top 144 isretracted (the opposite of the earlier extension) whilst the treatmentbeam 110 is administered. As the table top 144 is retracted, thedeflection of the table top 144 will reduce. The deflection profile thathas been determined by the processor is used to predict the changingvertical position of the location of the target region, according to thecurrent level of extension (or amount of retraction). Knowing that thetarget region is not moving (retracting) perfectly parallel to the axisof the bore 113 but is instead following a particular (e.g. bananashaped) profile relative to the axis of the bore allows the treatment tobe adjusted accordingly.

For example, the treatment can be offset in such a way that theradiation isocenter 124 can be kept precisely within the target regionover the full distance that the table top 144 moves along thelongitudinal axis 113 during the treatment. In one example, the verticalposition of the table top 144 can be raised or lowered along thevertical axis 111 to coordinate the absolute vertical position of thetable top 144 with the deflection profile so as to maintain the targetregion substantially at the isocenter and optimise the treatment byreducing the amount of healthy tissue exposed to harmful radiation. Thisvertical movement is actuated and controlled by the subject supportsurface 114 which, as described above, is configured to extend the tabletop 144 along the vertical axis 111. In other words, the treatment canbe correlated to, and using, the predicted deflection profile duringretraction of the table top 144, which is the reverse of the deflectionprofile determined during extension of the table top 144. The processorthat determines the deflection profile can be used to synchronise thetreatment according to the deflection profile or it can supply theinformation required to do so to another processor that is used tocontrol the treatment. In this way, it is possible to perform anaccurate spiral treatment whilst only taking initial images, rather thanduring the treatment. This reduces the harmful effects of the imaging,whilst also increasing the speed of the treatment.

In one example, a lung spiral treatment is performed using the disclosedapparatus. This treatment is performed by scanning a treatment beam 110over the target region, starting from towards the end of the patient 140that comprises the patient's head and moving down the patient's body fora distance of 500 mm. The scanning is achieved by physically moving thepatient 140, on the table top 144, through the treatment beam 110, asshall now be explained. In one example, the treatment beam is alsomoved. For example, a tilted beam can be used.

To get the patient 140 into position, the patient 140 first climbs ontothe table top 144 in a first, non-extended longitudinal position. Thetable top 144 is connected to the couch 114 by two connection points,also referred to as the table top attachment points 148 (although thisnumber may be greater or smaller). These two table top attachment points148 are fixed on the table top 144 so that the table top 144 is attachedby the same two points 148 in both the first position and a secondposition. However, the connections 148 to the subject support surface114 are movable from a first longitudinal position to a secondlongitudinal position relative to the subject support surface 114. Thislongitudinal movement can be achieved using any appropriate means. Forexample, the table top 144 may be connected to the couch 114 in such away as to be configured to move along it in a longitudinal directionalong a set of rollers, sliders, or along a rail. The movement along thelongitudinal axis 113 from a first position to a second position can bedriven manually but can also be driven by, for example, one or moreelectric motors or by any other suitable means.

The weight of the patient 140 on the table top 144, particularly if thepatient 140 is large, will cause the table top 144 to flex by an amount,for example, 3 mm. When the table top 144 has a subject 140 positionedon it, it is to be considered as in a loaded state. The amount of flexwill vary along the length of the table top 144, with the flex beinggreater further along the table top 144 away from the table topattachment points 148. However, once the patient 140 is on the table top144, if the patient does not move relative to the table top 144, theabsolute amount of flex of the table top 144 will not change over thecourse of the treatment or with the extension of the table top 144. Thetable top 144 may comprise additional means to prevent the patient 140from moving on it during the treatment or transport into the bore, forexample, straps, blocks, braces or other suitable means. In other words,the flex of the table top 144 as a whole does not change as the tabletop 144 is extended, but only the amount of flex measured relative to aparticular longitudinal point (i.e. relative to the sensor 146).

Before the patient 140 is positioned on the table top 144, the sensor146 is used to provide a first reference value or signal. This value orsignal and the subsequent values or signal generated by the sensor 146can be considered to be representative of the height of the table top144 at the position of the sensor 146 in an unloaded state. As thepatient 140 is positioned on the table top 144 in its first(non-extended) position (i.e. the loaded state), the table top 144flexes and the sensor 146 is compressed, resulting in an electric signalor a change in electrical signal that is communicated to the processorso that the processor can determine a new height of the table top 144and therefore, the amount of deflection caused by the particular patient140 in the first position along the table top 144. For example, thesensor 146 may determine that there has been a change in height of thetable top 144 (in other words a deflection) of 3 mm when comparing theheight of the table top 144 with the patient 140 on it in its firstposition, to the height of the table top 144 without the patient 140 onit in its first position.

In this example, the table top 144 is then extended along thelongitudinal axis 113 into the bore so as to position the inferior endof the target region beyond the location that will be the centre ofradiation during treatment. At this point, the location of the targetregion may be known approximately due to previous diagnostics. Theapproximate position target region may be physically marked on thepatient 140 by use of a pen or a tattoo. This can be used to assist thepatient 140 being moved to approximately the correct location forreceipt of the start of the treatment, for example, manually by anoperator. This can be assisted by the use of laser positioning. However,the position of the actual target region can move inside the patient 140relative to the marked position on the outside of the patient 140 due toswelling or for other reasons. In order to accurately know the positionof the target region at the time or nearer to the time of treatment andso that the patient can be precisely positioned and thereby reduce anyunnecessary exposure of healthy tissue to the treatment beam, thepatient 140 is scanned using an MR imaging apparatus 112 to obtaininitial CBCT images. This allows the precise location of the targetregion to be determined so that the couch 114 can then be adjustedfurther, for example by extension in a vertical 111, lateral 115 orlongitudinal 113 direction so as to precisely position the patient 140and particularly the target region, relative to the radiation isocenteror isocenter 124. For example, the target region is positioned within 3mm, 1 mm or 0.1 mm of the desired location. In one example, it isdesirable to have the target region within 1 mm of the intendedposition. The table top extension in this position will be referred toas the second (extended) position, although the second position may alsobe at a greater extension than that suitable for the start of thetreatment. Whilst the table top 144 is in the second position, thesensor 146 will provide a second height measurement for the table top144 which, when compared with the reference value, enables a deflectionof the table top 144 in the second position, at the location of thesensor 146, to be determined. This second deflection can be compared bythe processor to the first deflection to determine the change in flexbetween the first longitudinal position and the second longitudinalposition. Because in the second longitudinal position the sensor 146 ismeasuring the height of the table top 144 at the end of the table top144 that is closer to the table top attachment points 148, the amount offlex determined for the second longitudinal position is expected to beless than for the first longitudinal position.

Between the first position and the second position, the flex will varyby an unknown amount that will depend on how close the position is tothe attachment points 148 of the table top 144 and cannot easily becalculated without knowing the centre of mass of the subject 140, whichis something that will vary between subjects 140 and is hard todetermine. As a result, the sensor 146 is used to measure the height ofthe table top 144 at one or more positions (which correspond todifferent levels of extension of the table top 144 along thelongitudinal axis 113) between the first position and the secondposition. The first position is often the non-extended position and thesecond position is often the maximally extended position (for thattreatment). In this example, the sensor 146 measures the height of thetable top 144 for the first 1000 mm of extension, every 1 mm. The sensor146 data is then used to determine the amount of deflection every 1 mmand, in this way, a deflection profile is generated by the processor.This deflection profile can be generated during or after transport ofthe table top 144 into the bore (or patient 140 loading).

Having accurately positioned the patient 140 using the CBCT image andthe couch 114 and table top 144, the treatment is begun. The treatmentbeam 110 is turned on, as described previously. Either continuously asthe treatment beam is administered, or in one or more intervals betweenthe administering of the treatment beam, the table top 144 is retractedby a distance that corresponds to the desired length of the targetregion that is receiving the treatment beam, in this case, 500 mm. Thedeflection profile, saved in the memory associated with the processor,can be used to predict the amount of deflection of the table top 144 andtherefore the change in the vertical position of the target region, atany particular time/position of the retraction. For example, if thedeflection is 5 mm the vertical movement will raise the couch 114 or thetable top 144 by 5 mm to compensate for the deflection.

This allows the treatment to be optimised by compensating for thechanging height of the target region as the table top 144 is moved fromthe second longitudinal position back to the first longitudinal position(or some other position therebetween corresponding to the end of thetarget region and also referred to here as a third longitudinalposition). The treatment can be adjusted by the processor thatdetermines the deflection profile, or the deflection profile can becommunicated to a separate processor that is configured to adjust thevertical height according to the deflection profile.

Once the table top 144 has reached the third longitudinal position, thetreatment is stopped by turning the beam completely off. The table top144 is then fully retracted to the first longitudinal position to enablethe patient 140 to easily dismount from the couch 114.

In one example, the processor is also configured to take deflectionmeasurements whilst the table top 144 is being retracted. Thesemeasurements can be compared to those taken during the extension, to seeif the deflection has varied, which could be indicative of a patient 140having moved. By comparing the deflection at a particular point duringretraction of the table top 144 to the corresponding measurement takenat the same point during the extension of the table top 144, a change indeflection value can be calculated. In one example, the processor isconfigured to stop the radiation if the change in deflection value isgreater than a change in deflection safety threshold value. In oneexample, after the radiation has been stopped in response to the changein deflection value being greater than the change in deflection safetythreshold value, the processor is configured to instruct a CBCT scan toaccurately check the patient 140 position and, for example, recalibratethe position of the target region accordingly. In one other example, inresponse to the change in deflection value being greater than the changein deflection safety threshold value, the processor is configured toalter the treatment in such a way as to compensate for the change indeflection caused by, for example, the movement of a patient 140.

The sensor 146 that is measuring table top 144 position and flex needsto be fast and accurate. For example, the sensor can have an accuracy ofaround 0.1 mm or better to help ensure the tolerance is a reasonablelevel. As described above, an LVDT sensor can be used in which a magnetis moved inside a coil. LVDT don't have electronics near the sensor andare simple. As described above, the LVDT, for example an induSENSORLVDT, may comprise a pressure wheel or a standard roller that is builtinto the LVDT. Alternatively, a triangulation sensor or a proximitysensor can be used. Any other appropriate type of sensor could also beused as the sensor 146, as long as it is capable of accuratelydetermining the position or deflection of the table top 144. More thanone sensor 146 can also be used to provide redundancy, to increase thereliability of the measurements and the deflection profile, or for anyother reason. In one example, multiple sensors 146 are located along thelateral axis 115 of the couch 114 and are configured to determine adeflection of the table top 144 in the lateral direction. If multiplesensors 146 are used, the sensors 146 can be the same kind or could bedifferent from each other. In one example, the one or more sensors 146are chosen to be radiation hard to prevent damage from scatteredradiation from the linac and CBCT over time.

The table top 144 is made from one or more rigid materials such assteel, aluminium, titanium, a composite or any other suitable material.The table top 144 may also comprise a softer material such as a foam,designed to improve the comfort or support of the patient 140. The tabletop 144 may also comprise multiple layers, one of which includes aplastic. In one example, these materials are chosen to be radiation hardto prevent them from becoming damaged or brittle following repeatedexposure to emitted radiation.

Whilst the materials chosen for the table top 144 will be chosen toprovide an adequate degree of rigidity, the system disclosed allows thematerials use to be less rigid than would otherwise be required in aradiotherapy apparatus without table top 144 bending compensation. Thisis because the bending caused by a choice of less rigid materials cannonetheless be compensated for by adjusting the radiotherapy or othertreatment, as described above. This results in a wider range ofmaterials being suitable for use in the table top 144. For example,materials that are less rigid but cause less interference to theradiation can be used. This in turn reduces the amount of radiation thathas to be generated, thereby saving power and minimising the damage doneto any healthy tissue exposed to the radiation. Furthermore, rigidmaterials are often expensive and reducing the requirements for rigidityenables cheaper or more commonly available materials to be used for thetable top 144.

It is possible to mount more sensors to also measure the flex of themain body of the subject support surface 114, but this flex will be lessgreat than the table top flex. The structural flex will depend on thecouch 116/table top 144 position in the longitudinal direction 113.There will be a flex in the main structure of the couch 116 that isrelated to the rigidity of the materials used for its construction,which is why rigid materials are desirable. However, the majority of theflex still comes from the deflection of the table top 144. In oneexample, markers are added on to the table top 144 and this is thenmeasured with cameras. For example, the camera or cameras could beplaced on the floor and look upwards to the underside of the table top144, or could look from the side to determine the deflection. The cameraneed to process high resolution images to get the accuracy of themeasurements and the determination of the deflection to within a rangeof 0.1 mm. In another example, a C-Rad scanner system is used to measurethe deflection.

The processor may also be configured to use data from a memory thatstores information such as the dimensions and configuration of thecomponents so that these can be used in the calculations controlling themovement of the assorted components and to prevent, for example, thecouch 114 from colliding with the gantry 116.

It should be noted that the various embodiments may be implemented inhardware, software or a combination thereof. The various embodimentsand/or components, for example, components and controllers for these,also may be implemented as part of one or more computers or processorsor field-programmable gate arrays (FPGAs). The computer or processor orFPGA may include a computing device, an input device, a display unit andan interface, for example, for accessing the Internet. The computer orprocessor may include a microprocessor. The microprocessor may beconnected to a communication bus. The computer or processor or FPGA mayalso include a memory. The memory may include Random Access Memory (RAM)and Read Only Memory (ROM). The computer or processor or FPGA furthermay include a storage device, which may be a hard disk drive or aremovable storage drive such as an optical disk drive, and the like. Thestorage device may also be other similar means for loading computerprograms or other instructions into the computer or processor.

As explained above, the system allows treatments such as spiraltreatments to be performed accurately without the need for re-imagingthe patient 140 during the treatment (such as re-imaging the patient 140after the table top 144 has been partially retracted which may otherwisebe done). The system is also useful for long field treatments where atable shift is needed to be able to radiate the whole target regionbecause it increases the accuracy of the knowledge of the position of atarget region as the table top 144 is moved from one position toanother. Getting the couch 114 into the correct position directly beforeperforming the CBCT scan will save time, so table corrections will notbe needed after the CBCT scan. The system is also well suited forcouches 116 that do not have six degrees of freedom of movement.Re-imaging the patient 140 is harmful to the patient 140 and soalleviating the need for re-imaging reduces the harm done to the patient140 thereby improving his or her wellbeing. What is more, re-imaging mayrequire the movement of the table to be stopped and will add additionaltime and costs to the treatment. Removing the requirement for re-imagingtherefore increases the speed at which a treatment can be performed,thereby increasing patient 140 throughput and improving the efficiencyof the radiotherapy apparatus. The system also enables the position ofthe target region to be known with greater certainty and accuracy, whichenables the treatment to be performed with greater accuracy andconfidence in treating the target region. This also minimises theradiation received by healthy tissue.

1. A radiotherapy apparatus for delivering radiation to a subject, theapparatus comprising: a source of radiation configured to rotate aboutan isocenter and emit radiation in a radiation plane containing saidisocenter; a subject support surface configured such that at lease aportion of the subject support surface can be located substantially atthe isocenter; and a subject support surface rotation mechanismconfigured to rotate the subject support surface about an axis ofrotation that passes through the isocenter, wherein the subject supportsurface rotation mechanism is located outside the radiation plane. 2.The radiotherapy apparatus of claim 1, wherein the axis of rotation isat least one of a longitudinal axis, a transverse axis or a verticalaxis.
 3. The radiotherapy apparatus of claim 1, wherein the subjectsupport surface rotation mechanism is configured to rotate the subjectsupport surface before, after, or during a treatment.
 4. Theradiotherapy apparatus of claim 1, wherein the radiotherapy apparatusfurther comprises: a bore for receiving the subject.
 5. The radiotherapyapparatus of claim 1, wherein the subject support surface rotationmechanism comprises: at least one of a first curved guide or a secondcurved guide, wherein a center of curvature of at least one of the firstcurved guide or the second curved guide is located at a vertical axisthat passes through the isocenter.
 6. The radiotherapy apparatus ofclaim 5, wherein at least one of the first curved guide or the secondcurved guide is aligned within a horizontal plane that is perpendicularto a vertical axis.
 7. The radiotherapy apparatus of claim 5, comprisingthe first curved guide and the second curved guide, wherein the firstcurved guide and second curved guide are spaced apart from each other ina vertical direction.
 8. The radiotherapy apparatus of claim 7, whereinthe first curved guide and the second curved guide are aligned along avertical axis.
 9. The radiotherapy apparatus of claim 5, wherein atleast one of the first curved guide or the second curved guide areformed as first and second curved guide rails.
 10. The radiotherapyapparatus of claim 5, wherein at least one of the first curved guide orthe second curved guide are formed as first and second curved trenchguides.
 11. The radiotherapy apparatus of claim 9, wherein theradiotherapy apparatus comprises a gantry, wherein at least one of thefirst or the second guide rails are connected to the gantry.
 12. Theradiotherapy apparatus of claim 1, wherein the subject support surfacerotation mechanism is configured to rotate the subject support surface+/−40-20 degrees about the subject support surface axis of rotation. 13.The radiotherapy apparatus of any preceding claim, wherein the subjectsupport surface comprises: an extendable table top; and a sensorconfigured to measure a vertical position of the extendable table top,wherein the radiotherapy apparatus comprises: a processor configured to:determine a deflection of the extendable table top using the measuredposition; and control a treatment using the radiotherapy apparatusaccording to the deflection.
 14. A method for controlling a subjectsupport surface in a radiotherapy apparatus comprising a source ofradiation configured to rotate about an isocenter and emit radiation ina radiation plane containing said isocenter, the method comprising:providing a subject support surface configured such that a portion ofthe subject support surface can be located substantially at theisocenter; and rotating the subject support surface about an axis ofrotation that passes through the isocenter using a subject supportsurface rotation mechanism connected to the subject support surface,wherein the subject support surface rotation mechanism is locatedoutside the radiation plane. 15-27. (canceled)
 28. Acomputer-implemented method for controlling a radiotherapy apparatus,the method comprising: measuring a first vertical position of anextendable table top in an unloaded state at a first longitudinalposition of the extendable table top; measuring a second verticalposition of the extendable table top in a loaded state at he firstlongitudinal position of the extendable table top; determining adeflection of the extendable table top using the first vertical positionand the second vertical position; and controlling the radiotherapyapparatus according to the deflection.
 29. The computer-implementedmethod of claim 28, further comprising: moving the extendable table topin a loaded state top from the first longitudinal position to a secondlongitudinal position; measuring a third vertical position of theextendable table top in the second longitudinal position; determining asecond deflection of the extendable table top using the first verticalposition and the third vertical position; generating a deflectionprofile using the first deflection and the second deflection; andcontrolling a treatment using the radiotherapy apparatus according tothe deflection profile.
 30. The computer-implemented method of claim 29,further comprising: moving the extendable table top in a loaded statetop from the second longitudinal position to the first longitudinalposition; measuring a fourth vertical position of the extendable tabletop in the second longitudinal position; determining a change indeflection of the extendable table top at the first longitudinalposition using the second vertical position and the fourth verticalposition; and controlling the treatment using the radiotherapy apparatusaccording to the change in deflection.
 31. A non-transitorycomputer-readable storage medium comprising instructions which, whenexecuted by a processor of a computer, cause the processor to: measure afirst vertical position of an extendable table top of a radiotherapyapparatus in an unloaded state at a first longitudinal position of theextendable table top; measure a second vertical position of theextendable table top in a loaded state at the first longitudinalposition of the extendable table top; determine a deflection of theextendable table top using the first vertical position and the secondvertical position; and control the radiotherapy apparatus according tothe deflection.
 32. A radiotherapy apparatus for delivering radiation toa subject, the apparatus comprising: a source of radiation configured torotate about an isocenter and emit radiation in a radiation planecontaining said isocenter; and a subject support surface including aportion configured to be located substantially at the isocenter, thesubject support surface comprising: a subject support surface movementmechanism configured to rotate the subject support surface about an axisparallel to an axis that passes through the isocenter, wherein thesubject support surface movement mechanism is located outside of theradiation plane.